In U.S. Pat. No. 4,201,052 and copending application Ser. No. 117,936, filed Feb. 4, 1980, having a common assignee with the present application, there is disclosed hydraulic circuits which include a valve assembly, comprising a pilot operated meter-in valve and pilot operated meter-out valve, which is preferably mounted directly on an actuator.
In such a system when used in cranes and the like it is desirable to prevent drift when the load is held in an elevated position. The present invention is intended particularly to provide a hydraulic system of the above described type which will effectively prevent drift in such applications.
Basically, the invention comprises the above described hydraulic circuit including interposing a pilot operated check valve between the meter-out valve and the opening to one end of the actuator which is operable to permit flow or interrupt to the actuator and incorporates time delay means so that it closes after the meter-out valve closes. The pilot operated check valve also is operable to open before the meter-out valve.